Low voltage d.c. power sources are widely used in manufactures of many different kinds. Most known low voltage d.c. power supplies, when operated from a source of, for example, utility line voltage (e.g., 117 volts, 220 volts, etc.) employ the well known step-down transformer, rectifier, and usually feedback regulator circuit means so as to provide a d.c. voltage having good constancy and relative freedom from line voltage excursions. Such means are oftimes expensive and complex when compared to the product in which they are intended to find application. A real need exists, which my invention serves to satisfy, for an inexpensive, small, easy to mass-produce low d.c. voltage power supply which has good stability characteristics and high efficiency. In particular, the desirability for such a d.c. power supply is ever-increasing as solid-state controls, for example microprocessors or the like, reach into lower priced mass-produced consumer products.
In an earlier U.S. Pat. No. 3,355,650 Tolmie describes an arrangement for delivering a nominal d.c. value to a motor e.g., electric shaver or the like) from an a.c. mains which may vary. However, the inherent stability of the so-produced d.c. voltage is poor relative to what is required to satisfy more demanding electric loads, such as presented by control circuits including a microprocessor or the equivalent. This instability is described in the "SCR Manual Including Triacs and Other Thyristors", fifth edition 1972 publication ETRC-3875B by Semiconductor Products Department, General Electric Co., Syracuse, N.Y. 13201. Chapter 4.10, page 93 paragraph 2 particularly describes the phenomenon: "The transfer function is very non-linear and repeatibility of setting is not possible either with different SCR's or with temperature due to I.sub.GT variation." The "transfer function" describes the point where the SCR turns "on" when the only source of gate current is a substantial resistance coupled to the anode thereof. The class of triggering described therein is also that taught by Tolmie. In addition, the arrangement brings about a premature device failure mode whenever the SCR current rating is cost effectively near the a.c. load demand. The cause is well described in the "Thyristors, Rectifiers, and Diacs" databook SSD-206A 1973 edition published by R.C.A. Solid State Division, Box 3200, Somerville, N.J. 08876. Reference to page 438, paragraph 2 which effectively says that: " . . . it is always advantageous to provide a gate current pulse that has a magnitude exceeding the d.c. value required to trigger the device." While on page 442, paragraph 1 states: "Although the circuit is capable of providing variable power to the load, it is heavily dependent on the gate current distribution, and results in uncontrolled conduction angles for a given value of gate series resistance." The instant invention overcomes these earlier difficulties, through the novel inclusion of an abrupt breakdown voltage responsive means coupled directly between the anode and the gate of the s.c.r., or its equivalent, so as to provide rapid infusion of gate current once the threshold of the voltage responsive means is reached. This action provides a highly stable unipolar interrupter signal which is filtered as a source of constant d.c. potential, while at the same time serves to deliver the remaining part of the a.c. power cycle to the a.c. load work function without any important loss of effectiveness.
My invention's teachings derive the low d.c. voltage directly from the primary power line, thus there is no inherent isolation as might be provided by oridinary power supplies using transformers. The invention proposes, however, that in cost-effective products this presents no particular technical problem and any disadvantages are offset by cost savings, as line voltage operated relay controllers, timers, and the like have been used for years without difficulty. Furthermore radio, television, phonograph and other consumer oriented products are known to have employed direct connection between the a.c. primary line and the internal workings.
In the most basic expression of my invention merely five circuit elements, employed in novel arrangement, serve to acheive performance results which prior to my finding would require substantially more complicated and costly apparatus.